The present invention relates to a device for interchanging suction pipettes which can be removed from a magazine and coupled to a fitting head for the automatic fitting of printed circuit boards with component.
It is known to couple suction pipettes onto a fitting head for the automatic fitting of components on printed circuit boards, after removal from a magazine, and return them to the magazine from the fitting head after a fitting operation. The pipettes provided in the magazine are usually received by a sleeve having a star-shaped part that is connected by rotation in an interlocking manner to the intersecting point of the pipette and secured by a latching plate. This process is explained in greater detail in conjunction with FIG. 1 in which a fitting head 3; a rotary motor for the fitting head 3; a star-shaped latching plate 12; and a star-shaped part 4 arranged so as to be aligned with respect to each other on the drive shaft of the rotary motor 14. During the coupling of the pipette, the star-shaped part, which has a longitudinal passage 18, is inserted by exerting a pressure in the direction of the pipette (Z axis) into a star-shaped cutout in the pipette held in a magazine. The star-shaped cutout is situated on the surface of the pipette which faces the fitting head. By rotation of the star-shaped part 4 in the direction of the pipette while held firmly in the magazine, the star-shaped part 4 is locked in a bayonet-like manner to the pipette, where by the outer edges of the projection of the star-shaped part pass under the star-shaped cutout in the circumferential direction. This corresponds to the locking position between the star-shaped part 4 and the pipette. A star-shaped, resilient latching plate is fastened congruently to the star-shaped part 4 and has a projection or a nub which faces the pipette. On rotation of the star-shaped part 4 together with the latching plate into the locking position, the nub slides over an arc of a circle outside the star-shaped cutout of the pipette until it enters the locking position, i.e. a notch which is situated on the arc of the circle. This has the effect of maintaining the locking position. An annular seal, which is provided on the star-shaped part ensures a tight connection between the above-mentioned hole in the star-shaped part 4 and the pipette, so that a vacuum can be applied to the pipette via the hole in the star-shaped part 4.
One problem with such a locking arrangement, i.e., between the star-shaped part and the pipette, is that a relatively large force is required for detaching the star-shaped part from the pipette because of the latching force of the nub of the latching plate. This is so, even if the notch in the arc of the circle has sloping surfaces. Thus, the force or moment required for rotating the star-shaped part in relation to the pipette therefore has to be relatively large. Conversely, if a certain, minimum moment is specified, the retaining force is defined by this moment. A further disadvantage is that the wear on the notch and nub is relatively great, and so the useful life of the pipettes is limited.
The object of the present invention is to provide a device, whereby the interchange of suction pipettes on a fitting head can be carried out reliably with comparatively small coupling and uncoupling moments. An important aspect of the present invention which enables this object to be realized is the fact that a relatively small moment is sufficient for rotating the star-shaped part into the locking position when a pipette is picked up out of a magazine, since unlike the prior art, the latching plate does not directly engage the pipette itself, but rather a special sliding ring having a low coefficient of friction. Accordingly, during the latching and unlatching process, the nub of the star-shaped latching plate does not have to overcome sides of the notch of the pipette. Because the nub slides on the sliding ring which has a small coefficient of friction, after the locking position is reached and the latching plate is relieved of load, the nub is gently inserted into the notch in the Z axis from above from the sliding ring. Conversely, on insertion of the pipette into a magazine and with the latching plate being pressed down, the nub is moved upward in the Z axis out of the notch because the latching plate comes to rest on the sliding ring. As a result, the moments occurring during the latching and unlatching of the nub can advantageously be reduced by up to a factor of six (6) and there is no wear in the region of the notch. In a preferred embodiment of the present invention, the retaining force between the pipette and star-shaped part is increased by prestressing of the latching plate.
The low coupling and uncoupling moments enable a motor which is less powerful in terms of torque and therefore lighter in weight to be used. This is of importance since the star-shaped part and latching plate and the pipette connected thereto in the locking position fit directly on the shaft of the motor, and hence the motor can be accelerated directly in the direction of the Z axis during the fitting process. When placing components on a printed circuit board using relatively low attaching forces, the weight of the motor must also be low. By means of the present invention, an attaching force of, for example, 0.5 N, which is the lowest which is required, can be achieved with a relatively high fitting speed. Further, the novel device may be designed in such a manner that it can also be used in conjunction with presently known pipettes and a new design for the pipettes is not required.